Microwave switches that are activated by an electrical control signal are often undesirable. For example, when such a switch is used in conjunction with certain types of transmission line or various types of antenna structure, the wire or other type of conductor that supplies the switch control signal sometimes must be positioned within a region of the transmission line or antenna that supports propagation of the microwave signal. In these situation, the control conductor can interfere with the desired propagation, radiation or reception of the switched microwave signal. Further, signals that are induced in such conductors by extraneous electromagnetic fields can result in undesirable system noise or interference.
Various optically actuated or controlled switches have been proposed to eliminate the need for conductors or wires that carry control signals to the switch. For example, attempts have been made to use various types of depletion-layer photodiodes as microwave switches (e.g., p-i-n diodes, p-n junction diodes, Schottky barrier diodes, and heterojunction diodes). Although satisfactory in some applications, the current-voltage characteristics of a depletion-layer photodiode are not symmetric about the current and voltage axes. Thus, a bias signal must be supplied to a depletion-layer photodiode if the device is to be used as a microwave switch for sinusoidal or other time-varying signals that have both positive and negative components.
One type of optically activated microwave switch that has found some application utilizes a laser-excited conductive plasma or a solid-state photoconductive material. For example, switches have been proposed in which a gap of approximately 2-25 micrometers is formed in a microstrip transmission line. When the gap is illuminated by a high-power laser pulse, the gap is short-circuited and microwave signals propagate along the microstrip transmission line in the normal manner. When the gap is not illuminated by the laser pulse, the gap forms a high impedance discontinuity in the transmission line.
The need for high energy illumination pulses makes such microstrip gap switches unsuitable for many applications. For example, it can be necessary to simultaneously activate a large number (e.g., an array) of switches from a single source of optical energy. If the source of optical energy is a single laser diode or other such device, the amount of peak optical power that may be available for each device may be 1 milliwatt or less. Attaining such high optical sensitivity requires both sensitive devices and efficient coupling of the optical signal. In this regard, it is preferable that the optically sensitive region of a microwave switch match the pattern of light that is supplied to activate the switch. By way of example, in systems in which light is coupled from the end of an optical fiber to the switching device, the spot of light provided will be substantially circular in geometry and typically will range in diameter from approximately 10 microns (single-mode optical fiber) to approximately 50 microns (multi-mode optical fiber).
Although various proposals have been made, the prior art has not provided a highly sensitive optoelectronic microwave switch that does not require electrical bias or control signals while simultaneously meeting design constraints normally associated with microwave switching devices. Specifically, to be suitable for widespread application, a microwave switch must exhibit low insertion loss when activated (i.e., a relatively low on-state resistance such as 50 ohms or less) and must exhibit relatively high isolation when not activated (a high off-state impedance). As is known in the art, microwave switch circuits that include or consist of a semiconductor circuit present capacitive reactance that often limits the maximum frequency at which the switch can be employed. For example, in a 50 ohm system, a microwave switch must present an off-state capacitance of approximately 16 femto-farads (16.times.10.sup.-15 farads) to maintain an isolation of 20 decibels (1000 ohms reactance) at a frequency of 10 gigahertz.